A storage system is equipped with some functions for improving convenience, like snapshot. To perform the snapshot function, control information such as difference information and generation management information is required. In recent years, enhancement for the snapshot function has been desired, such as increase in the number of obtainable generations, increase in the capacity for application, and reduction in size of difference to be obtained. For such functional enhancement, a huge amount of control information is required.
On the other hand, there are cases where a sufficient access performance is required for control information. For example, in an access to control information tied to a user's I/O (Input/Output) request like difference data in the snapshot function, if the access performance to the control information is low, the user's I/O request cannot be processed within a practicable time period.
For this reason, control information requiring a high-speed access is stored in a high-speed memory such as a cache memory or a local memory in some systems. However, since the capacity of a memory to be mounted in a storage system has an upper limit, the amount of control information to be stored in the memory has a limitation. Accordingly, functional specifications for the snapshot, such as the number of generations, the applicable capacity, and the obtainable size of difference data, are limited.